LLVM JIT: Use shortcuts for constants and types in IR emission code

The IRBuilder class allows easy creation of constants and integer
types.
LLVM JIT: Recompile LW

LW is currently implemented identically to the cached interpreter's
version, so a memory accessor function is still called if the load
comes from RDRAM.

Four new global variables are required:
* readmem, which is the array with all the word-sized accessors;
* address, which tells the memory accessors what to load (N64);
* rdword, which tells the memory accessors where to store what was
  loaded (native);
* skip_jump, which is part of N64 exception handling logic.
LLVM JIT: Combine the implementations of similar opcodes

This reduces code duplication.
LLVM JIT: Recompile all widths of aligned loads

The new opcodes are LB, LBU, LH, LHU, LWU and LD.
They have minimal code duplication among each other and with LW.

They are currently implemented identically to the cached interpreter's
version, so a memory accessor function is still called if the load
comes from RDRAM.

Three new global variables are needed: readmemb, readmemh and readmemd,
the arrays containing memory accessors for 8-bit, 16-bit and 64-bit
loads.